Apparatus and method for performing a handover in a wireless communication system

ABSTRACT

A method of performing a handover in a wireless communication system employing a macro base station and a femto base station is provided. The method comprising: monitoring an uplink signal transmitted from a user equipment to the macro base station; transmitting a handover initiation message to the macro base station on the basis of a result of comparing the uplink signal&#39;s strength obtained by the monitoring with a reference value; and performing a handover of the user equipment from the macro base station to the femto base station. According to the present invention, there is provided a method of initiating a handover by a femto base station by fining user equipments located in a cell coverage of the femto base station. In addition, a signaling overhead is reduced by utilizing previously existing information such as uplink transmit power.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to an apparatus and method for performing a handover in awireless communication system.

BACKGROUND ART

With the development of communication and the spread of multimediatechnology, a wireless communication system uses various techniques formassive data transmission. A method of assigning a more number offrequency resources may be used as a method of increasing radiocapacity. However, since the number of frequency resources is limited,there is a limitation when the more number of frequency resources areassigned to a plurality of users. As one of methods for effectivelyutilizing the limited frequency resources, there is a method of creatinga small-sized cell. When the small-sized cell is created, one basestation (BS) provides a service to a less number of users, and thus theBS can assign a more number of frequency resources to the users. Bycreating the small-sized cell, a service for massive data transmissionwith better quality can be provided to the plurality of users.

To increase system efficiency and to improve quality of service (QoS) inan indoor environment, a protocol standardization work for a wirelessaccess system supporting a femto-cell is in progress in the institute ofelectrical and electronics engineers (IEEE) 802.16 task group whichdefines a next generation wireless interface standard and in theworldwide interoperability for microwave access (WiMAX) forum which isan non-profit organization providing a service and a network protocolfor an IEEE 802.16-based broadband wireless access system. In the WiMAXforum, a femto BS is defined as a low-power low-cost BS connected withan Internet protocol (IP) network through a fixed wireless link or alocal broadband wired link. The femto BS is connected to the IP networkwidely used in homes or offices, and provides a mobile communicationservice by accessing a core network of a mobile communication system.That is, the femto BS may be connected to the core network of the mobilecommunication system through a digital subscriber line (DSL). A user ofthe mobile communication system may receive a service via a conventionalmacro-cell in an outdoor environment, and may receive a service via afemto-cell in an indoor environment. The femto-cell improves indoorcoverage of the mobile communication system by correcting a problem inwhich the conventional macro-cell provides a deteriorating serviceinside a building. Since the femto-cell can provide a service only to aspecific designated user, a voice service and a data service can beprovided with high quality. Further, the femto-cell can provide a newservice not provided by the macro-cell. With the wide use of thefemto-cell, fixed-mobile convergence (FMC) can gain a momentum, andindustry-based costs can be reduced.

In general, a cell coverage of the femto BS is smaller than a cellcoverage of the macro BS, and the cell coverage of the femto BS may beentirely or partially included in the cell coverage of the macro BS. Ina case where a user equipment (UE) belonging to the cell coverage of themacro BS performs communication, if the UE also belongs to the cellcoverage of the femto BS, it may be advantageous to perform a handoverfrom the macro BS to the femto BS. The same also applies in the oppositecase. A handover may be initiated by the UE, the macro BS, or the femtoBS. A handover initiated by the UE is a UE-initiated handover. Ahandover initiated by the macro BS is a BS-initiated handover.

The UE-initiated handover may be performed by the UE by directlymeasuring a signal of a neighbor BS and transmitting to the macro BS amessage for requesting a handover to a desired BS. On the other hand,the BS-initiated handover may be performed by the macro BS bytransmitting a handover command message to the UE.

The BS-initiated handover may also be performed by the femto BS. In thiscase, the femto BS directly scans neighbor UEs and allows a desired UEto initiate a handover from the macro BS to the femto BS. However, amethod of performing a handover initiated by the femto BS has not beenclearly defined up to now. Accordingly, there is a need for an apparatusand method for performing a handover in a wireless communication systememploying a macro BS and a femto BS.

DISCLOSURE Technical Problem

The present invention provides an apparatus and method for initiating ahandover by a femto base station when a user equipment is handed overfrom a macro base station to the femto base station.

Technical Solution

According to an aspect of the present invention, a method of performinga handover in a wireless communication system employing a macro basestation and a femto base station is provided. The method comprising:monitoring an uplink signal transmitted from a user equipment to themacro base station; transmitting a handover initiation message to themacro base station on the basis of a result of comparing the uplinksignal's strength obtained by the monitoring with a reference value; andperforming a handover of the user equipment from the macro base stationto the femto base station.

According to other aspect of the present invention, an apparatus forperforming a handover in a wireless communication system is provided.The apparatus comprising a processor for monitoring an uplink signaltransmitted from a user equipment to a macro base station, fortransmitting a handover initiation message to the macro base station onthe basis of a result of comparing the uplink signal's strength obtainedby the monitoring with a reference value, and for performing a handoverof the user equipment from the macro base station to a femto basestation.

Advantageous Effects

According to the present invention, there is provided a method ofinitiating a handover by a femto base station by fining user equipmentslocated in a cell coverage of the femto base station. In addition, asignaling overhead is reduced by utilizing previously existinginformation such as uplink transmit power.

DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of a WiMAX system structure supporting afemto-cell.

FIG. 2 shows an example of a frame structure.

FIG. 3 shows a scenario in which a UE performs a handover from a macroBS to a femto BS according to the present invention.

FIG. 4 is a flowchart for explaining a method of performing a handoverinitiated by a femto BS according to an embodiment of the presentinvention.

FIG. 5 is a view for explaining a process of transmitting an uplinksignal by a UE to monitor a femto BS according to an embodiment of thepresent invention.

FIG. 6 is a view for explaining a process of transmitting an uplinksignal by a UE to monitor a femto BS according to another embodiment ofthe present invention.

FIG. 7 is a block diagram showing an apparatus for performing a handoveraccording to the present invention.

MODE FOR INVENTION

Embodiments of the present invention will be described hereinafter withreference to the accompanying drawings so that the invention can beeasily implemented by those skilled in the art. The present inventioncan be implemented in various different forms and is not limited to theembodiments described herein. Throughout the drawings, parts irrelevantto the present invention are omitted for clarity, and the same referencenumbers will be used to refer to the same or like parts. Further,various details that can be easily understood by those skilled in theart will be omitted.

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc.The UTRA is a part of a universal mobile telecommunication system(UMTS). 3rd generation partnership project (3GPP) long term evolution(LTE) is a part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPPLTE uses the OFDMA in downlink and uses the SC-FDMA in uplink. IEEE802.16m is an evolution of IEEE 802.16e.

FIG. 1 shows an example of a WiMAX system structure supporting afemto-cell.

Referring to FIG. 1, a typical wireless communication system includes auser equipment (UE) and a base station (BS). The wireless communicationsystem can be widely deployed to provide a variety of communicationservices, such as voices, packet data, etc.

The UE may be fixed or mobile, and may be referred to as anotherterminology, such as a mobile station (MS), an advanced mobile station(AMS), a user terminal (UT), a subscriber station (SS), a wirelessdevice, etc. The BS is generally a fixed station that communicates withthe UE and may be referred to as another terminology, such as anadvanced base station (ABS), a node-B, a base transceiver system (BTS),an access point, etc. There may be one or more cells within the coverageof the BS.

The BS may be classified into a femto BS and a macro BS according tocell coverage or deployment. In general, the femto BS operates within apermitted frequency range, and may use the same frequency band as themacro-cell or may use a frequency band different from that used in themacro-cell. A cell coverage of the femto BS is smaller than a cellcoverage of the macro BS. The cell coverage of the femto BS may overlapentirely or partially with the cell coverage of the macro BS. As such, astructure in which a small-ranged cell is repetitively positioned in awide-range cell is referred to as a hierarchy cell structure. The femtoBS may also be referred to as other terminologies, such as a femto-cell,a home node-B, a closed subscribed group (CSG), a WiMAX femto accesspoint (WFAP), etc. To distinguish from the femto-cell, a cell of themacro BS may also be referred to as a macro-cell.

A UE accessing the femto BS is referred to as a femto UE. A UE accessingthe macro BS is referred to as a macro UE. When the femto UE is handedover to the macro BS, the femto UE may become the macro UE. When themacro UE is handed over to the femto BS, the macro UE may become thefemto UE.

The femto BS can establish a broadband connection over the Internetprovided by an Internet service provider (ISP) by using a digitalsubscriber line (DSL), a cable, an optical fiber, a wireless connection,etc. The femto BS may be connected through the Internet to an accessservice network (ASN) of a femto-cell management system and a mobilecommunication system. The femto-cell management system may perform aregistration, authentication, and security procedure or the like of thefemto BS so that the femto BS can access a connectivity service network(CSN) of the mobile communication system.

A femto BS for an open subscriber group (OSG) operates like the macro BSwhen calling a UE. A femto BS for a closed subscriber group (CSG) maybroadcast a paging message only for a UE belonging to a correspondingCSG. Only permitted UEs can attempt an access to the CSG femto BS. TheOSG and CSG for the support of the femto BS may be defined in referenceto ‘system description document (SDD) of the IEEE 802.16 Task Group m’.

A downlink represents a communication link from a BS to a UE, and anuplink represents a communication link from the UE to the BS. In thedownlink, a transmitter may be a part of the BS, and a receiver may be apart of the UE. In the uplink, the transmitter may be a part of the UEand the receiver may be a part of the BS.

FIG. 2 shows an example of a frame structure. This structure may be aframe structure of at least one of a macro-cell and a femto-cell in ahierarchy cell structure.

Referring to FIG. 2, a superframe includes a superframe header (SFH) andfour frames F0, F1, F2, and F3. Although it is shown that eachsuperframe has a size of 20 milliseconds (ms) and each frame has a sizeof 5 ms, the present invention is not limited thereto. The SFH may belocated at a front-most position of the superframe. A common controlchannel is assigned to the SFH. The common control channel is used totransmit information regarding frames constituting the superframe orcontrol information (e.g., system information) that can be commonlyutilized by all UEs within a cell. A synchronization channel fortransmitting a synchronization signal may be deployed either inside oradjacent to the SFH. The synchronization signal may represent cellinformation such as a cell identifier (ID).

One frame includes a plurality of subframes SF0, SF1, SF2, SF3, SF4,SF5, SF6, and SF7. Each subframe can be used for uplink or downlinktransmission. Each subframe may consist of 6 or 7 OFDM symbols, but thisis for exemplary purposes only. Time division duplexing (TDD) orfrequency division duplexing (FDD) may be applied to the frame. In theTDD, each subframe is used in uplink or downlink transmission at thesame frequency and at a different time. That is, subframes included in aTDD-based frame are divided into an uplink subframe and a downlinksubframe in a time domain. In the FDD, each subframe is used in uplinkor downlink transmission at the same time and at a different frequency.That is, subframes included in an FDD-based frame are divided into anuplink subframe and a downlink subframe in a frequency domain. Uplinktransmission and downlink transmission can be simultaneously performedwhile occupying different frequency bands. Each subframe may include asubframe header. The subframe header may include radio resourceallocation information of a subframe.

Hereinafter, a handover performed by a femto BS will be described.

FIG. 3 shows a scenario in which a UE performs a handover from a macroBS to a femto BS according to the present invention.

Referring to FIG. 3, a femto BS 20, a UE-1 31, and a UE-2 32 areincluded in a cell coverage of a macro BS10. The UE-1 31 is alsoincluded in a cell coverage of the femto BS 20. Both of the UE-1 31 andthe UE-2 32 perform communication through a radio interface of the macroBS10.

If it is more advantageous for the UE-1 31 to perform communicationthrough a radio interface of the femto BS 20, the UE-1 31 may perform ahandover from the macro BS10 to the femto BS 20. The handover from themacro BS10 to the femto BS 20 may be performed in a UE-initiated manneror a BS-initiated manner.

In particular, a process of initiating a handover by the femto BS willbe described. If S1 denotes a signal transmitted by the UE-1 31 and S2denotes a signal transmitted by the UE-2 32, the signals S1 and S2 canbe sensed not only by the macro BS10 which is an actual receiving entitybut also by the femto BS 20 since both of the signals S1 and S2 aretransmitted in a radial direction according to a characteristic of aradio signal. The femto BS 20 monitors a signal transmitted by each UEto find a UE belonging to the cell coverage of the femto BS 20. By usingsignal strength obtained for the signals S1 and S2 as a result of themonitoring, the femto BS 20 can obtain locations of the UE-1 31 and theUE-2 32, more exactly, a distance between the femto BS 20 and the UE-131 and a distance between the femto BS 20 and the UE-2 32.

If the UE-1 31 exists in the cell coverage of the femto BS 20 and thusthe femto BS 20 determines that a handover is necessary, the femto BS 20may request a handover of the UE-1 31 to the macro BS10.

As such, the handover initiated by the femto BS 20 may include amonitoring process, a process of determining whether a handover isnecessary, and a process of requesting a handover of the UE to the macroBS10.

The above processes will be described hereinafter in greater detail inreference to the accompanying drawings.

FIG. 4 is a flowchart for explaining a method of performing a handoverinitiated by a femto BS according to an embodiment of the presentinvention.

Referring to FIG. 4, a macro BS transmits scheduling information to a UE(step S100). The scheduling information includes not only resourceallocation information regarding uplink transmission and downlinktransmission of the UE but also information such as an uplinktransmission period or transmit power. The UE transmits an uplink signalto the macro BS on the basis of the scheduling information (step S110).The femto BS monitors the uplink signal transmitted from the UE to themacro BS (step S120). Monitoring is a process of measuring receivedsignal quality such as received signal strength indication (RSSI). Inorder for the femto BS to perform monitoring, negotiation needs to befirst achieved with the macro BS for the uplink signal to be monitored.

For example, the uplink signal to be monitored may be uplink data. Theuplink data may include a header field as shown in the following table.

TABLE 1 Length Name (bit) Description Type 3 The type of PHY channelreport header is defined in Table 7. PREFERRED-DIUC 4 Index of the DIUCpreferred by the MS. UL-TX-POWER 8 UL Tx power level in dBm for theburst that carries this header (11.1.1). The value shall be estimatedand reported for the burst. UL-HEADROOM 6 Headroom to UL maximum powerlevel in dB, for the burst that carries this header, from 0 to 63 in 1dB steps. Should the headroom exceed 63 dB, the value 63 shall be used.The value repoirted shall represent the difference between the maximumoutput power and the maximum power trans- mitted during the burst.Reserved 1 Set to zero. CID 16 MS basic connection identifier. HCS 8Header check sequence (same usage as HCS entry in Table 5).

Referring to Table 1, a physical channel report header field includes avariety of information such as type information, a preferred downlinkinterval usage code (DIUC), uplink (UL) transmit (Tx) power, ULheadroom, etc. Herein, the UL Tx power is information indicating a powerlevel used by the UE in uplink transmission and is represented in dB.

To monitor the uplink data, the femto BS has to know a UE group which isa group of UEs to be monitored by the femto BS, and also has to knowscheduling information allocated to the UE group. Therefore, when themacro BS transmits the UE group information and scheduling informationin advance to the femto BS through a backbone network (this process isnot shown in FIG. 4), the femto BS can monitor an uplink data signal ofthe UE group. Further, the macro BS may instruct the UE to insert theheader field including the UL Tx power to the uplink data (this processis not shown in FIG. 4), and such an instruction may be transmitted byusing the scheduling information. The scheduling information may also bereferred to as a UL medium access control (MAC) message, a UL grantmessage, a UL MAP, or a physical downlink control channel (PDCCH).

For another example, the uplink signal to be monitored may be uplinkcontrol information. The uplink control information may be anyinformation indicating a state of a downlink channel, such as channelquality information (CQI), a fast feedback channel (FFBCH), a periodicranging message, an uplink reference signal, and a physical uplinkcontrol channel (PUCCH). The uplink control information may betransmitted in a periodic manner according to a persistent allocationscheme. Therefore, the femto BS can know a transmission time of theuplink control information without the use of additional schedulinginformation, and thus monitoring can be effectively performed. In thiscase, the macro BS may instruct the UE to transmit the uplink controlinformation in a periodic manner and according to pre-determined Txpower.

If the uplink control information is the periodic ranging message, theperiodic ranging message may be a message depending on dedicatedperiodic ranging dedicatedly allocated to the UE. In this case, thefemto BS may monitor a ranging preamble in a dedicated ranging slot ofthe macro BS.

The femto BS transmits a handover initiation message to the macro BSaccording to a result obtained by comparing the uplink signal's strengthobtained as a result of the monitoring with a reference value (stepS130).

For one example, the reference value is a Tx power value included in theuplink data. In this case, the Tx power value is equal to the UL Txpower of the header field included in the uplink data as shown in Table1 above. Therefore, the femto BS reads out the Tx power value from theuplink data and compares this value with the uplink signal's strength.Then, by using a difference between the uplink signal's strength withthe reference value, the femto BS can obtain a distance between thefemto BS and the UE. According to the distance, the femto BS candetermine whether the UE exists in the coverage of a femto-cell. If itis determined that the UE is located within the coverage of thefemto-cell, the femto BS transmits to the macro BS a handover initiationmessage for requesting initiation of a handover of the UE.

For another example, the reference value may be UL Tx powerpre-determined between the UE, the femto BS, and the macro BS. In thiscase, the femto BS predicts a distance between the femto BS and the UEby comparing the uplink signal's strength with the pre-determined UL Txpower. If it is determined that the UE is located in the coverage of thefemto-cell, the femto BS transmits to the macro BS the handoverinitiation message for requesting initiation of a handover of the UE.

For another example, the reference value may be a pre-determinedhandover threshold known to the UE, the femto BS, and the macro BS. Thatis, if the uplink signal's strength is greater than the handoverthreshold, it is determined that the UE exists in the coverage of thefemto-cell and thus the handover initiation message is transmitted. Onthe other hand, if the uplink signal's strength is less than thehandover threshold, it is determined that the UE does not exist in thecoverage of the femto-cell and thus a handover-related operation is notperformed.

The handover initiation message may be a message for instructing the UEto perform a scanning operation for finding a neighbor femto BS.Therefore, when the macro BS receives the handover initiation message,the macro BS transmits to the UE a scan request message for requestingthe UE to scan the neighbor femto BS (step S140). The UE performsscanning (step S150), and reports to the macro BS a candidate femto BSto which the UE intends to be handed over (step S160). The handover isprepared on the basis of the candidate femto BS.

The UE is handed over from the macro BS to the femto BS (step S170).This step of performing a handover may be achieved in such a manner thatthe femto BS transmits a handover request message to the macro BS, andin response thereto, receives a handover response message from the macroBS.

As described in the above explanation, there is an advantage in that ahandover is more effectively performed in a sense that negotiation onmonitoring becomes clear between the femto BS and the macro BS, and thefemto BS can initiate a handover of the UE by directly monitoring asignal of a neighbor UE.

FIG. 5 is a view for explaining a process of transmitting an uplinksignal by a UE to monitor a femto BS according to an embodiment of thepresent invention. In this case, the uplink signal is a fast feedbackchannel.

Referring to FIG. 5, a macro BS instructs the UE to transmit a fastfeedback channel (FFBCH) in a frame #k3 according to Tx power P3pre-determined or defined by the macro BS. By considering a delay of abackbone network and to provide a sufficient time to the femto BS forpreparation of monitoring, the instruction is performed in a frame #L(L<<k3) located ahead of the frame #k3. If the uplink signal is asounding channel, the macro BS announces in advance to the femto BS asequence used for the sounding channel.

FIG. 6 is a view for explaining a process of transmitting an uplinksignal by a UE to monitor a femto BS according to another embodiment ofthe present invention. In this case, the uplink signal is a sequenceand/or a message.

Referring to FIG. 6, a macro BS instructs the UE to transmit thesequence and/or the message instead of a fast feedback channel in aframe #k3 according to Tx power P3 pre-determined or defined by themacro BS. The instruction may be determined by the macro BS or may beperformed in a pre-agreed Manner. Herein, the sequence may be used as apilot for a decoding message. The message may include informationregarding Tx power of the uplink signal. By considering a delay of abackbone network and to provide a sufficient time to the femto BS forpreparation of monitoring, the instruction is performed in a frame #L(L<<k3) located ahead of the frame #k3. The macro BS may report to thefemto BS that the sequence and/or the message will be transmitted in theframe #k3.

FIG. 7 is a block diagram showing an apparatus for performing a handoveraccording to the present invention.

Referring to FIG. 7, an apparatus 50 for performing a handover includesa processor 51, a memory 52, a radio frequency (RF) unit 53, a displayunit 54, and a user interface unit 55. Layers of a radio interfaceprotocol are implemented in the processor 51. The processor 51 providesa control plane and a user plane. A function of each layer can beimplemented in the processor 51.

The processor 51 monitors an uplink signal transmitted from a UE to amacro BS, transmits a handover initiation message to the macro BS on thebasis of a result of comparing the uplink signal's strength obtained bythe monitoring with a reference value, and performs a handover of the UEfrom the macro BS. The memory 52 is coupled to the processor 51 andstores an operating system of the UE, applications, and general files.The display unit 54 displays a variety of information of the UE and mayuse a well-known element such as a liquid crystal display (LCD), anorganic light emitting diode (OLED), etc. The user interface unit 55 canbe configured with a combination of well-known user interfaces such as akeypad, a touch screen, etc. The RF unit 53 is coupled to the processor51 and transmits and/or receives radio signals.

All functions described above may be performed by a processor such as amicroprocessor, a controller, a microcontroller, and an applicationspecific integrated circuit (ASIC) according to software or program codefor performing the functions. The program code may be designed,developed, and implemented on the basis of the descriptions of thepresent invention, and this is well known to those skilled in the art.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

1. A method of performing a handover in a wireless communication systememploying a macro base station and a femto base station, the methodcomprising: monitoring an uplink signal transmitted from a userequipment to the macro base station; transmitting a handover initiationmessage to the macro base station on the basis of a result of comparingthe uplink signal's strength obtained by the monitoring with a referencevalue; and performing a handover of the user equipment from the macrobase station to the femto base station.
 2. The method of claim 1,wherein the uplink signal is uplink data.
 3. The method of claim 2,wherein the uplink data comprises a header field, and the header fieldcomprises the reference value.
 4. The method of claim 3, wherein thereference value is a power value used by the user equipment to transmitthe uplink signal.
 5. The method of claim 1, wherein the uplink signalis uplink control information.
 6. The method of claim 5, wherein theuplink control information is transmitted periodically.
 7. The method ofclaim 6, wherein the uplink control information is a ranging requestmessage transmitted periodically.
 8. The method of claim 5, wherein theuplink control information is channel quality information (CQI)indicating a state of a downlink channel.
 9. The method of claim 5,wherein the uplink control information is transmitted on a fast feedbackchannel.
 10. The method of claim 5, wherein the uplink controlinformation is an uplink reference signal.
 11. The method of claim 1,further comprising, before performing the monitoring, receivingscheduling information comprising resource information related to theuplink signal from the macro base station, wherein the uplink signal ismonitored by using the scheduling information.
 12. The method of claim1, wherein the reference value is a value pre-agreed among the femtobase station, the macro base station, and the user equipment.
 13. Anapparatus for performing a handover in a wireless communication system,the apparatus comprising a processor for monitoring an uplink signaltransmitted from a user equipment to a macro base station, fortransmitting a handover initiation message to the macro base station onthe basis of a result of comparing the uplink signal's strength obtainedby the monitoring with a reference value, and for performing a handoverof the user equipment from the macro base station to a femto basestation.
 14. The apparatus of claim 13, wherein the femto base stationprovides a service to a closed subscriber group (CSG).
 15. The apparatusof claim 13, wherein the processor performs monitoring on the basis of auser equipment group provided by the macro base station, and the userequipment group comprises user equipments connected to the macro basestation.